3 Answers
3

A simple pulse lengthener will work here.

It works like this:

The first block gets powered from the pressure plate (or you can power it in some other way), the redstone line immediately takes power from there, so all the pistons go up instantly. After some time the repeater draws power from the block and powers the next block, and so on...

When you step off the pressure plate, the first block no longer powers the redstone line, but all the next blocks still do; the first repeater turns off after some time and the 2nd block is not powered anymore... but the line is powered until the last block loses power.

If the signal is shorter than 4 ticks, this scheme won't work well, but buttons always give a 5-tick pulse and pressure plates' signal lasts at least 5 ticks.

And by the way, I used iron blocks, but these can be any non-transparent blocks,

Old answer

Or, for a different behavior, use an RS NOR Latch as in this screenshot.
This scheme will keep the pistons up for at least the time it takes the current to go through the repeaters, or for more time if the player keeps standing on the pressure plate.

If I may ask is it possible to extend the length of the deactivation using this method?
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Casey WeedAug 23 '12 at 17:09

@CaseyWeed Of course, just add more repeaters
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BlaXpiritAug 23 '12 at 17:09

1

Your new solution, the pulse lengthener, seems to be the ideal solution. It instantly activates, then deactivates after a short period of time. What's great is it's easy to build and adding a longer line of repeaters doesn't seem to cause any funny behavior.
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Casey WeedAug 23 '12 at 18:14

3

I was skeptical of this at first, then tested it. Works as advertised. The crux is that the iron blocks power the redstone, but the redstone can't power the iron blocks, preventing it from latching.
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MBraedleyAug 23 '12 at 20:51

1

I tried this with wooden blocks and it also works great.
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Casey WeedAug 24 '12 at 1:22

@BlaXpirit - I don't see how that fixes it.
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user9983Aug 23 '12 at 17:42

It won't "turn off, then turn on, then have a delayed off". Note that all the repeaters have the same direction.
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BlaXpiritAug 23 '12 at 17:44

@BlaXpirit I didn't notice the repeaters have different delays. I had considered this, but to make it work best, you'd need a cascade of 1-tick delays. I didn't mention it because it is impractically large.
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user9983Aug 23 '12 at 17:47

I've tried this and it seems like it just might be my best bet. It has a good enough delay and even if it might be large when scaled up, at least it works reliably and is simple enough.
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Casey WeedAug 23 '12 at 18:01

One way to delay the deactivation of a signal is to add a parallel path of activation that contains repeaters to create the delay AND has a long enough length of redstone wire on the end (i.e. greater than 15 blocks) such that it can't reactivate itself. Here's an example:

I think my friend actually came up with this by happenchance and it worked, but it was very large to build up a delay long enough for you to walk through. I'll have to try this and modify it.
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Casey WeedAug 23 '12 at 17:10